In recent years, data volume handled by computers is dramatically increasing, and there is a growing need for a high-capacity storage system.
As a high-capacity storage system, there is a hierarchical storage system that combines a high-speed low-capacity storage unit having short required access time, but low capacity, and a low-speed high-capacity storage unit having long required access time but with high capacity. In this hierarchical storage system, an appropriate data arrangement is significant, placing highly accessed data in the high-speed, low-capacity storage unit, and infrequently accessed data in the low-speed, high-capacity storage unit, so that a user's time required for accessing can be reduced.
Considering the above discussed point, Japanese Patent Laid-open Publication No. H05-12077 (hereinafter, referred to as “Patent Document 1”), for instance, discloses the following technique.
Patent Document 1 discloses a technique that obtains access frequency based on an access history of data that is stored in multiple storage units each having different performance and capacity, and places frequently accessed data into a storage unit having a high accessing speed, and infrequently accessed data into a storage unit having a low accessing speed.
However, in this technique, there is a problem in that when a particular data item is accessed, there is a high probability that other data item having relevance to this particular data is accessed. Even when the particular data item is accessed, if this other data item is accessed only infrequently, it takes a long time for accessing, since the other data item is placed in a low-speed storage unit.
In order to solve the above problem, Japanese Patent Laid-open Publication No. H08-263335 (hereinafter, referred to as “Patent Document 2”) discloses a technique as follows: When the other data item is accessed within a certain period of time after the particular data item is accessed, it is assumed that the particular data item and the other data item are belonging to an identical group, and when any of the data within the group is accessed, the other data within the group are placed in the high-speed storage unit.